refactor(joinir): Pattern 4 modularization with CarrierInfo/ExitMeta

Removes hardcoded "sum" and ValueId(15) from Pattern 4 lowerer by
introducing CarrierInfo and ExitMeta structures.

Changes:
- New carrier_info.rs: CarrierInfo, CarrierVar, ExitMeta structs
- loop_with_continue_minimal.rs: Returns (JoinModule, ExitMeta)
- pattern4_with_continue.rs: Dynamic binding generation from metadata

Design approach: "Thin meta on existing boxes" (ChatGPT proposal)
- CarrierInfo: Built from variable_map, not AST re-analysis
- ExitMeta: Carrier name + JoinIR ValueId pairs from lowerer
- LoopExitBinding: Auto-generated from CarrierInfo + ExitMeta

Test: loop_continue_pattern4.hako outputs 25 (unchanged)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/join_ir/lowering/mod.rs

@@ -15,6 +15,7 @@
 //! - `if_select.rs`: Phase 33 If/Else → Select lowering
 //! - `if_dry_runner.rs`: Phase 33-10 If lowering dry-run スキャナー（箱化版）
 
+pub mod carrier_info; // Phase 196: Carrier metadata for loop lowering
 pub mod common;
 pub mod exit_args_resolver;
 pub mod funcscanner_append_defs;